The three-dimensional distributions on the ribosome of initiation factors and of large and small subunit proteins and RNA, will be determined. Subunits will be reacted with antibodies directed against specific ribosomal proteins, against initiation factors and elongation factors or against specific regions of RNA and observed by electron microscopy. Unique three dimensional sites for each protein or factor will be mapped by comparing images of subunits (with attached antibodies) in different orientations. Our objective will be to relate the three dimensional distribution of RNA and protein to ribosomal function. The small subunit is responsible for recognizing the initiation site on mRNA with the participation of initiation factors and fmet-tRNA, for binding aminoacyl tRNAs, for associating with the large subunit, and for regulating the translational fidelity of messenger reading. The large subunit binds the acceptor stem of aminoacyl tRNAs entering the A site; catalyzes peptidyl transfer, and participates in elongation and translocation. By relating the three dimensional distributions of ribosomal proteins, factors and regions of RNA's with known biochemical information, we will attempt to elucidate the structural aspects of the molecular events occurring during protein synthesis. Comparative studies of eukaryotic ribosome structure will also be pursued in order to relate common structural features of eukaryotic and prokaryotic ribosomes to common functions in protein synthesis.